Compact aluminium alloy heat treatment method

ABSTRACT

The invention relates to a method for the heat treatment of a moving aluminium alloy strip, the aluminium strip has an upper-surface and a lower-surface, the method comprising moving the aluminium strip over at least two rotating heating rolls, wherein the heating rolls comprises an outer-surface, such that a surface of the aluminium strip is in heat-transfer contact with the outer-surface of the heating rolls to induce heat into the aluminium strip to heat the aluminium strip at an annealing temperature, and comprising moving the aluminium alloy strip over a first rotating heating roll followed by moving the aluminium strip over a second rotating heating roll such that alternating the upper-surface and the lower-surface of the aluminium strip are in heat-transfer contact with the outer-surface of the rotating heating rolls.

FIELD OF THE INVENTION

The invention relates a method and a compact apparatus for the heat treatment of an aluminium alloy strip.

BACKGROUND OF THE INVENTION

Aluminium alloys are used extensively for various purposes, such as automotive components, structural components, and many other uses. Traditionally, aluminium alloys are either direct chill cast or continuously cast. Often, an ingot, slab, or strip is rolled to a final gauge that is deliverable to a customer (e.g., automotive manufacturer or part processing plant). In some cases, the aluminium alloy may need to undergo some sort of thermal treatment to achieve desirable temper properties. For example, annealing can improve formability of an aluminium article and solution heat treatment followed by a quench can improve strength of the aluminium article.

To achieve high volume throughput, aluminium alloy articles can be continuously annealed or solution heat-treated in large continuous processing line. Traditionally, such a continuous processing line occupies a very large building and requires expensive and complicated equipment. For example, such a continuous annealing solution heat-treatment line requires passing an aluminium alloy strip through numerous sections to sufficiently raise the temperature of the aluminium strip to keep it at solution heat treatment temperature followed by quenching sometimes require a processing line of up to 130 meters or longer. As these continuous processing lines also include additional operations, such as entry sections, devices to stitch or weld strips together, loopers, tension controllers, degreasing before the annealing or solution heat treatment, and other metallurgical or surface treatment operations after quenching and final recoiling, the total developed length of these continuous processing lines can reach up to 800 meters or longer. Low tension must be maintained while the aluminium strip is moving at high temperatures and in the quench section, and to avoid surface defects the aluminium strip has to be maintained without contact with any surrounding equipment or structures in these sections. In practice this is achieved by the use of forced air applied on the two surfaces of the aluminium strip to keep it appropriately suspended in the air. If the aluminium strip makes physical contact with equipment or structures, it may damage the equipment or structure, as well as damage the surface of the aluminium strip, necessitating a shutdown and scrapping of the damaged aluminium strip, as well as any aluminium strip in the up to 130 meter or longer annealing or solution heat treatment and quench sections that is affected and any aluminium necessary to start up a new processing ran (e.g., another 800 meters or more). Additionally, to maintain desired temperatures, the forced air used to suspend the aluminium strip must be heated as well in the annealing or solution heat treatment section.

Annealing and solution heat treatment involves heating and cooling the aluminium article to specific temperatures and holding at those temperatures for specific durations of time. The temperature-time profile of an aluminium article can greatly affect the resulting strength, ductility and other overall properties (e.g. resistance to crash for automotive body sheets) of the aluminium article. In some cases, for example for the AA6XXX and AA7XXX-series aluminium alloys widely used in automotive and transportation applications, annealing or solution heat treatment and quenching of aluminum alloys can involve heating the article at a high temperature until alloying elements (mainly silicon and magnesium for the AA6XXX-series alloys, and zinc, magnesium and optionally copper for the AA7XXX-series alloys) are dissolved in solid solution in the metal article, then quenching the metal article to lock these elements in a supersaturated solid solution. After annealing or solution heat treatment and quench, the aluminium can he hardened by progressive recombination and precipitation of the alloying elements in the aluminium matrix. This hardening can take place at room temperature (e.g., naturally aged) for a duration, or result from a duration at a slightly elevated temperature (e.g., artificially aged or pre-aged, typically in the range of 70° C. to 200° C.), and/or from further processing (e.g., cleaning, pretreatment, coating, or otherwise). The painting operation of an automotive body and its paint-curing-cycle is an example of such further processing step contributing to hardening of the aluminium alloy.

This solution heat treatment and quench is also of interest for aluminium alloys which do not harden by precipitation, for example the AA5XXX-series aluminium alloys, which mainly harden by solid solution of magnesium, where the heating-up helps delivering and controlling a recrystallized structure, and the holding at time and temperature to control the size of the recrystallized grains. The degree of recrystallization and the grain size directly impact mechanical properties, surface aspect, and the yield point elongation (YPE) in particular for AA5XXX-series alloys.

Similarly, for aluminium alloys hardened by precipitation, AA2XXX, AA6XXX and AA7XXX-series alloys for example, increasing the heat-up rate to annealing or solution heat treatment helps delivering and controlling a recrystallized grain structure in the aluminium strip, and the holding at time and temperature to control the size of the recrystallized grains. The degree of recrystallisation, the texture of the material and the size of the recrystallized grains directly impact the forming ability of the aluminium strip.

In practice with state of the art equipment for continuous heat treatment of aluminium strips, the heat up speed to solution heat treatment is limited by the fact that the heating is done by the flow of air also suspending the moving aluminium strip, thus considerably reducing the possibility to accelerate this heat up speed.

Another issue associated with state of the art equipment for continuous heat treatment of aluminium strips available is the tendency of the aluminium strip to deform during the solution heat treatment (or more generally treatment at elevated temperature) in the sections of the furnace where heat up and mainly holding at maximum temperature take place. A typical pattern of deformation is a flat M-like shape or sea-gull shape along the transverse section, which may bring the aluminium strip in contact with the nozzles supplying the air which holds and suspends the aluminium strip in position during heat up and soaking at elevated temperature. This may create unacceptable defects on the surface of the aluminium strip and in some cases it may cause its breakage, generating major production stoppages.

In addition, this M-shape or other surface deformation occurring during annealing or solution heat treatment (or more generally treatment at elevated temperature) makes the quenching operation more difficult when the quenching operation employs water or any other liquid. A pocket or valley on the surface of the aluminium strip will create locally a potential accumulation of water or any other liquid making the cooling heterogeneous and enhancing aluminium strip deformation during the quenching operation.

The rapid cooling after annealing or solution heat treatment also plays an important role. A too slow cooling will allow a portion of the alloying elements to leave from the solid solution and no more contribute to the further hardening. These may also precipitate at the grain boundary and weaken the strength of the aluminium alloy by initiating premature failure at the grain boundary, therefore decreasing the material performance, its crash resistance in case of for example AA6XXX-series alloys. From that perspective, the cooling should be maximized, but with the state of the art equipment for continuous heat treatment of aluminium strips, maximizing the cooling means cooling the strip with a spray or a mist of water which creates deformations of the aluminium strip. This deformation is an issue as its amplitude may be big enough to create contact between the strip and the equipment such as the air nozzles of the devices supplying the air maintaining the aluminium strip in position in the quench section. The described deformation and risk of contact generally increase with the increase of cooling, obliging in practice to compromise between fast cooling and acceptable deformation.

From this it follows that the state of the art industrial equipment available on the market for continuous annealing or solution heat treatment and quenching of aluminium strips do not provide full satisfaction. Due to the slow heat up speed with hot air, their annealing or solution heat treatment sections are long and costly in investment. The equipment is limited in their possibility to apply a quick heat up to annealing or solution heat treatment temperature as well as a high cooling speed during the quenching operation, both desirable for various metallurgical reasons. They also generate strip distortion which contribute to heterogenous quenching when water or another liquid is used, which may create surface defects due to strip interaction with the equipment and even major strip breaking in production. The operation of such continuous heat treatment lines remains in practice difficult and very costly.

Several improvements have been proposed in the art to remediate the weaknesses of such lines.

Patent document WO-2016/037922-A1 discloses a method for the continuously annealing of aluminium sheet of the AA6XXX-series, wherein before or near the entry section of the continuous annealing furnace the aluminium sheet is pre-heated, preferably inductively, to a temperature of 5° C. to 100° C. below the set solution heat-treatment temperature of 500° C. to 590° C. Patent document WO-2016/091550-A1 discloses a method for the continuously annealing of aluminium sheet of the AA7XXX-series, wherein before or near the entry section of the continuous annealing furnace the aluminium sheet is pre-heated, preferably inductively, to a temperature of 5° C. to 100° C. below the set solution heat-treatment temperature of 370° C. to 560° C. Patent document WO-2018/064228-A1 proposes a compact continuous heat treatment line having a short heating zone for rapidly heating metal strip to a solutionizing temperature using magnetic rotors, such as permanent magnetic rotors. The magnetic rotors can be used to levitate the metal strip within a gas-filled chamber. And patent document WO-2018/064145-A1 discloses a non-contact heating apparatus using a series of rotating magnets to heat, levitate, and/or move metal articles therethrough.

But none of these solutions addresses in full the weaknesses of the state of the art equipment available on the market for continuous annealing or heat treatment and quenching of aluminium strips.

DESCRIPTION OF THE INVENTION

As will be appreciated herein below, except as otherwise indicated, aluminium alloy and temper designations refer to the Aluminium Association designations in Aluminum Standards and Data and the Registration Records, as published by the Aluminium Association in 2018 and are well known to the persons skilled in the art. The temper designations are laid down also in European standard EN515.

For any description of alloy compositions or preferred alloy compositions, all references to percentages are by weight percent unless otherwise indicated.

The term “up to” and “up to about”, as employed herein, explicitly includes, but is not limited to, the possibility of zero weight-percent of the particular alloying component to which it refers. For example, up to 0.25% Cu may include an aluminium alloy having no Cu.

It is an object of the invention to provide a compact method and a corresponding apparatus for the heat treatment of an aluminium alloy strip at an annealing or solution heat treatment temperature.

This and other objects and further advantages are met or exceeded by the present invention providing a method for the continuous heat treatment of a moving aluminium alloy strip, the aluminium alloy strip has an upper-surface and a lower-surface, the method comprising moving or transporting the aluminium alloy strip over at least two rotating heating rolls, wherein the heating rolls comprise an outer-surface such that a surface of the aluminium alloy strip is in heat-transfer contact with a part of the outer-surface of the heating roll to induce heat into the aluminium alloy strip to heat the aluminium strip to an annealing temperature, and comprising moving the aluminium alloy strip over a first rotating heating roll followed by moving the aluminium alloy strip over a second rotating heating roll such that alternating the upper-surface and the lower-surface of the aluminium alloy strip are in heat-transfer contact with the outer-surface of the rotating heating rolls. The aluminium alloy strip is heat treated by heating it to a predefined annealing temperature, by this is meant a temperature at which the aluminium sheet is annealed or solution heat treated.

The method of the invention requires a compact apparatus for the heat treatment of aluminium alloy strip. The aluminium alloy strip is moved over at least two cylindrical rotatable heating rolls to bring the aluminium alloy strip to a required and predefined annealing temperature and to control the soaking time at the annealing temperature.

The method and the apparatus can be provided with speed control means (i.e. the uncoiling speed of the aluminium alloy strip from a coil and the rotational speed of the heating rolls) and strip tension control means to control the heat transfer from the cylindrical rotatable heating roll to the aluminium alloy strip. The rotational speed of each cylindrical heating roll is individually adjustable.

To ensure that at least the upper-surface and the lower-surface of the aluminium alloy strip is brought into heat transfer contact with the outer-surface of the heating rolls at least two heating rolls are provided such that the aluminium alloy strip moving over a first heating roll such that the upper-surface of the aluminium alloy strip is in heat transfer contact with the first heating roll followed by moving the aluminium alloy strip over a second heating roll whereby the lower-surface of the aluminium alloy strip is in heat transfer contact with the outer-surface of the second heating roll to ensure as much as possible a fast and homogeneous heat-up of the aluminium sheet. Alternatively, firstly the lower-surface is in contact with the first heating roll followed by the upper-surface of the aluminium alloy strip in contact with the outer-surface of the second heating roll. The first cylindrical heating roll is rotatable in a first direction, i.e. clockwise or counter-clockwise, and the second cylindrical heating roll is rotatable in an opposite second direction.

Moving or transporting an aluminium alloy strip over a cylindrical rotatable roll may result in some plastic deformation at the surface of aluminium alloy the strip if the stress seen at the surface of the strip exceeds the yield stress. The advantage of alternating the aluminium alloy strip over at least two rotatable heating rolls moving in opposite directions is also that the both surfaces of the aluminium alloy strip are deformed, the resultant is that the effect is symmetrical. Furthermore, it results in improved flatness control of the aluminium alloy sheet.

The heat-transfer to an aluminium alloy strip resulting from the direct contact with the outer-surface is much more effective than the heating of an aluminium alloy strip with heated air as is done in industrial scale continuous annealing lines. A series of experiments have shown that for 1 mm sheet material of the AA6016-series the time to reach from ambient temperature a solution heat-treatment temperature of 540° C. is less than 30 seconds when heated from 1 side and about 10 to 15 seconds when heated from both sides by keeping it in direct contact between two metal blocks having a temperature of 540° C. By increasing the heat input, for example by using an additional external heat source like induction heating, this heat-up time can be reduced to less than 10 seconds. Whereas in an industrial scale continuous annealing time the required heat-up time to this solution heat-treatment temperature is typically in a range of 45 to 55 seconds. This considerable reduction of heat up time in the method of this invention results amongst others in a better and more homogeneous grain recrystallization due to the increased heat-up speed of the aluminium alloy strip and to a significant reduction in the size of the required equipment to achieve this effect.

It is an important aspect of the invention that the aluminium alloy strip is in direct contact with the rotatable heating rolls, i.e. there is thermal contact between the aluminium alloy strip and the outer-surface of the rotatable heating roll. In the prior art, the direct contact of a moving aluminium alloy strip at elevated temperature against any static part of the equipment is to be avoided as it may lead to undesirable surface damaging of the aluminium alloy strip. However, in accordance with this invention it has been found that by selecting the right surface coating of the rotatable heating roll surface, damaging does not need to be an issue as there is tangent contact without any differential speed difference between the aluminium alloy strip and the outer surface of the rotatable heating roll other than the aluminium alloy strip expansion during heat-up.

Nevertheless, the number of cylindrical rotatable heating rolls in the method of this invention should be limited, and preferably two or three, but not more than four, heating rolls are employed to keep the system as compact as possible.

In an embodiment, the outer-surface of the rotatable heating rolls is coated with wear resistant material with a high thermal conductivity and a low coefficient of friction. In a preferred embodiment, the outer-surface of the rotatable heating rolls is uniformly coated with a composite diamond coating, for example the commercially available Composite Diamond Coatings of the Series-1100 from Endure Coatings can be used. The overall low coefficient of friction of 0.10 to 0.20 combined with a high hardness of more than 1,000 Vickers, and typically around 1,200 Vickers, contributes in combination with strip tension and speed control, to limiting the occurrence of surface defects on the moving aluminium alloy strip.

This example of a suitable coating is non-limiting and the use of other technologies, in particular thermal spraying, including high velocity oxygen fuel spraying (HVOF), to provide a heat conductive, wear resistant and highly adhesive coating onto the surface of the heating rolls is envisaged by this invention. Other suitable materials include ceramic coatings, such as titanium nitride, tungsten carbide, chromium nitride, and the like.

A rotatable heating roll is preferably manufactured from a metal selected from the group of cast iron, steel, stainless steel, cemented carbides, copper, copper-based alloy, and aluminium based alloy, with a sufficient compressive strength and wear resistance so as to undergo only elastic deformations during the operation of the method. It can be heated by various heating means, for example by resistance heating, e.g. with a set of heaters positioned inside the rotatable roll together with temperature measurement and temperature control means. The power supply can be made for example via a connection through the axis of the cylindrical rotatable heating roll. The choice of the heating roll material can also be such as to obtain an effective heating via induction heating means. This can be of particular interest for at least the first rotatable heating roll in a set of heating rolls, where a significant heat input is required between the aluminium alloy strip and the rotatable heating roll. This can be achieved from the inside of the rotatable heating roll, but alternatively or in addition thereto also from inductors positioned perpendicular to the outer-diameter of a rotatable heating roll.

In an embodiment, the inductive heating means are provided contributing directly to the heating up of the aluminium alloy strip itself, or even before the aluminium alloy strip is in direct contact with the outer-surface of the heating roll. This would result in an effective and fast heat-up of the aluminium alloy strip and in reducing the required contact time with the outer-surface of the cylindrical rotatable heating roll(s).

In an embodiment, the aluminium alloy strip is moving or transporting while one surface is in heat-transfer contact with a rotating heating roll and the other surface of the aluminium strip is facing a thermal shield or screen to modulate heat-loss. The thermal shield or screen consists of a wall(s) or a roof structure made from a material that is ideally reflective on the side facing the heating roll and the aluminium sheet, e.g., a stainless-steel plate. The thermal shield or screen is to reduce heat-loss of the moving aluminium alloy strip over the outer-surface of the rotating heating roll by reflecting the infra-red radiation of the aluminium alloy strip or by adsorbing and re-emitting infra-red radiation. The thermal shield or screen also prevents or limits uncontrolled temperature loss by preventing air currents around the moving aluminium sheet in the space or chamber defined by the rotating heating roll and the thermal shield or screen.

More preferably the thermal shield or screen further comprises active heating means for improved temperature control of the moving aluminium alloy strip. The active heating can be done in various ways, in particular the heating is selected from the group consisting of infrared, radiant-tube, gas-fired heating, direct resistance, induction heating, and combinations thereof. In an embodiment, the active heating means are provided separate from the thermal shield to induce heat into the aluminium alloy strip in addition to the heat input from the aluminium alloy strip while in heat-transfer contact with the outer-surface of the heating roll.

To make the contact between the aluminium alloy strip and the outer-surface of the rotatable heating roll(s) more effective, a leveller can be provided.

To control the tension of the moving aluminium alloy strip while in contact with the outer-surface of the rotating heating roll(s), a tension controller, e.g. as in regular in the art, can be provided between the leveller and the entry section of a first rotatable heating roll.

In an embodiment, and similarly as with regular industrial methods and equipment for the continuous annealing and quench of aluminium alloy strips, the entry section of the apparatus or facility to perform the method of this invention can be equipped or provided with an un-coiler for handling the aluminium alloy strip out of a coil coming from the rolling mill; with one or more devices to join together the ends the aluminium alloy strips from different and successive coils, for example by means of stitching or friction stir welding; and with loopers sized to apply the previous operations without substantially lowering the overall speed of the heat-treatment line; and also with one or more degreasing sections removing prior to temperature heat up residues of rolling lubricant or rolling lubricant burn from the surface of the aluminium alloy strip to avoid that such residues would pollute the equipment and the surface of the aluminium alloy strip.

In an embodiment, the aluminium alloy strip following the heat treatment is rapid cooled or quenched to below about 100° C., and preferably to below 50° C., and more preferably to ambient temperature.

The quenching can be achieved by conventional devices with the benefit that the aluminium strip exits the rotatable heating rolls substantially flat, making the control of the quenching and its homogeneity over the aluminium strip much easier than current industrial equipment for continuous annealing or solution heat-treatment and quenching of aluminium alloy strips.

In an embodiment, the aluminium alloy strip following the heat treatment is rapid cooled or quenched to below about 100° C. by moving the aluminium alloy strip over at least one rotatable cooling roll, and preferably a set of two or more rotatable cooling rolls, wherein the rotatable cooling roll comprises an outer-surface, such that a surface of the aluminium alloy strip is in heat-transfer contact with the outer-surface of the cylindrical rotatable cooling roll to remove heat from the aluminium alloy strip to rapidly cool the aluminium strip at a temperature below about 100° C., and preferably to below 50° C., and more preferably to ambient temperature. The rotatable cooling roll(s) can be water cooled to achieve a high degree of heat transfer. The cooling rolls can be made from the same material and provided with the same or similar surface coating as the rotatable heating rolls. By using one or more rotatable cooling rolls, a more homogenous cooling of the aluminium alloy strip is obtained resulting in significantly less distortion. Also the undesired formation of a so-called seagull shape or M-like shape across the width of the aluminium alloy strip is avoided. The cooling speed of a 1 mm gauge AA6016 aluminium alloy strip is typically in a range of about 100-200° C./sec from annealing temperature when using water cooled cooling rolls made from aluminium.

Optionally the aluminium strip is further cooled by actively spraying water, water-based emulsions, water mist, or another cooling medium on the surface of the aluminium strip not in contact with the outer-surface of the cooling roll(s) to accelerate the heat removal. In a preferred embodiment, the spray of fine water droplets mostly evaporates at the surface of impact of the aluminium alloy strip. This additional cooling can be performed on the first cooling roll but can also be performed on the second and any further cooling rolls. In this way, rapid cooling speeds of for example 200-400° C./sec be reached or even higher such that for example a 1 mm gauge aluminium alloy strip with minimum strip distortion is obtained. Minimum strip distortion is here defined as low enough to be fully removed by passing the strip through a conventional leveller.

Optionally the cooling by means of the cylindrical rotatable cooling rolls can be supplemented by active cooling of the outer-surface of the aluminium alloy strip not in contact with the outer-surface of the cooling roll(s) via pressurised air, e.g. using one or more arrays of air-nozzles.

Following the cooling step, the usual processing steps can be applied similar as with the state of the art equipment for continuous annealing or solution heat treatment and quenching of aluminium strips. These operations include levelling, exit looper, shearing and recoiling. These can also include surface treatment (for example in sequences of degreasing, rinsing and etching), coating (for example a passivation layer can be applied), lubrication of the aluminium strip in perspective of further stamping and forming operations, and some dedicated thermal cycles like pre-ageing. The aluminium alloy strip may remain coiled or can be cut-to-length.

Next the heat-treated aluminium alloy strip can be formed in a forming operation. It can be any forming operation used to shape three-dimensional components, and includes in particular operations like stamping, deep drawing, pressing, superplastic forming, press forming, and roll forming, or combinations thereof.

In an embodiment, the annealing temperature is in a range of 400° C. to 590° C. As is well known to the skilled person, the annealing temperature is alloy dependent. For aluminium alloys of the AA5XXX-series the annealing temperature is typically in a range of about 400° C. to 540° C., and preferably of about 470° C. to 540° C. For aluminium alloys of the AA6XXX-series the annealing temperature is typically in a range of about 500° C. to 590° C., and preferably of about 510° C. to 580° C. For aluminium alloys of the AA7XXX-series the annealing or solution heat-treatment temperature is typically in the range of about 400° C. to 560° C. For the AA7XXX-series alloys having a purposive addition of Cu (i.e. Cu>0.25%) the temperature is typically in a range of about 400° C. to 530° C., and preferably of about 450° C. to 520° C., and for the AA7XXX-series alloys having no purposive addition of Cu (i.e. Cu<0.25%) the temperature is typically in a range of about 400° C. to 560° C., and preferably of about 470° C. to 530° C.

In an embodiment, the aluminium alloy sheet has a thickness in the range of about 0.3 mm to 4.5 mm, preferably of about 0.7 mm to 4 mm, and more preferably of about 0.8 mm to 4 mm. The sheet width is typically in the range of about 600 to 2700 mm.

In an embodiment, the aluminium alloy strip has a composition within the AA2XXX-, AA5XXX, AA6XXX- or AA7XXX-series aluminium alloys. In a preferred embodiment the aluminium alloy is within the AA6XXX-series aluminium alloys, and includes, but is not limited to, 6005, 6009, 6010, 6111, 6014, 6016, 6022, 6029, 6451, 6061, 6181, 6082, and 6182. In another embodiment, the aluminium alloy is within the AA5XXX-series aluminium alloys, and includes, but is not limited to, 5050, 5051, 5052, 5454, 5754, 5456, 5182, and 5083.

The aluminium alloy strip obtained by the method according to this invention can be used in automotive applications and other transportation applications, including aircraft and railway applications. For example, the disclosed resultant aluminium alloy products can be used to prepare automotive structural parts, such as bumpers, side beams, roof beams, cross beams, pillar reinforcements (e.g., A-pillars. B-pillars, and C-pillars), inner panels, outer panels, side panels, inner hoods, outer hoods, or trunk lid panels. The resultant aluminium alloy products and methods described herein can also be used in aircraft or railway vehicle applications, to prepare, for example, external and internal panels, including fuselage panels. Certain aspects and features of the present disclosure can provide metal articles with improved surface qualities and metallurgy, which can result in improved bonding capability and formability, which may be especially desirable for any of the applications mentioned herein, as well as others. The resultant aluminium alloy products and methods described herein can also be used in electronics applications. As non-limitative example, the resultant aluminium alloy products and methods described herein can be used to prepare housings for electronic devices, including mobile phones and tablet computers. In some examples, the resultant aluminium alloy products can be used to prepare housings for the outer casing of mobile phones (e.g., smart phones), tablet bottom chassis, and other portable electronics.

In a further aspect of the invention there is provided an apparatus or facility for carrying out the method as herein described and claimed, the apparatus comprising:

-   -   a heating-section comprising two or more rotatable heating rolls         adapted to move or transport in use an aluminium alloy strip         while in heat-transfer contact with the outer-surface of the         rotatable heating roll to induce heat into the aluminium alloy         strip to heat the aluminium alloy strip at an annealing         temperature;     -   a rapid cooling or quenching section for rapid cooling or         quenching of the aluminium alloy strip from the annealing         temperature to below 100° C., and preferably comprising at least         one rotatable cooling roll;     -   optionally one of more heat shields or screens with an interior         reflective surface to modulate the heat-loss of a moving         aluminium alloy strip and being positioned to face the side of         the aluminium alloy strip that is not in heat transfer contact         with the outer-surface of a rotatable heating roll;     -   optionally one of more heating means to induce heat into the         aluminium alloy strip in addition to the heat input into the         aluminium alloy strip while in heat-transfer contact with the         outer-surface of a rotatable heating roll;     -   optionally a tension controller is leveller is provided; and     -   optionally a tension controller is provided between the leveller         and the entry section of a first rotatable heating roll.

DESCRIPTION OF THE DRAWING

The invention shall now be described with reference to the appended drawings, in which:

FIG. 1 is a schematic representation of the principle of this invention;

FIG. 2 is a schematic representation of an exemplary method and apparatus;

FIG. 3 is a schematic representation of another exemplary method and apparatus; and

FIG. 4 is a schematic representation of another exemplary method and the apparatus.

FIG. 1 is a schematic representation of the principle of the method according to the invention. An aluminium alloy strip 1 is moving or transported in the direction of the arrows and is heat treated by bringing it in heat-transfer contact with the outer-surface of in this case three rotatable heating rolls 6,7,8. The required diameter of the rotatable heating rolls can be first order estimated by the following guidelines where:

v is the speed (m/sec) of the moving aluminium alloy strip;

Tc is the time (sec) of the required contact between the aluminium alloy strip 1 and the outer-surface of the heating rolls to heat-up the aluminium alloy strip to the required annealing or solution heat-treatment temperature and the required soaking time at this temperature. This is aluminium alloy dependent and can be established by simple experiments or thermodynamic computer modelling calculations by the skilled person;

Lc is the total length (m) of the required contact;

Di is the diameter (m) is heating toll number i;

Ki is the contact factor (dimensionless) of heating roll number i. Depending on the relative positions of the heating rolls and the position of the aluminium alloy strip, Ki is the ratio between the perimeter of heating roll i in contact with the strip divided by the total perimeter of said heating roll;

N is the number of rolls;

Lc=v·Tc or Lc=Di·Ki·π;

If it is assumed for this model calculation that all rolls have the same Ki and the same diameter, then this can be simplified to:

Lc=N·D·K and consequently D=Tc·v/(N·π·K)

To provide a first order of magnitude where Tc is 15 sec, v is 1 m/sec, N is 3 and K is 0.75, this would result in a heating roll diameter of 2.12 meter for each of three heating rolls.

The skilled person will immediately recognise that this concerns a mere model calculation and that various variations are possible or required while using the same principles.

In practice the diameter of a set of heating rolls can be varied within this set, but typically the diameter of each heating roll is in a range of about 1 meter to 3 meters.

FIG. 2 is a schematic representation of an embodiment of the method according to the invention and the apparatus employed therein. In this configuration, aluminium alloy strip 1 with a lower-surface 2 and an upper-surface 3 is being uncoiled from a coil 4 and moved or transported to a heating section comprising three cylindrical rotatable heating rolls 6,7,8 and followed by a rapid cooling section comprising of cylindrical three rotatable cooling rolls 9,10,11 and subsequently re-coiled into a coil 5. In the heating section, the upper-surface 3 of the aluminium alloy strip 1 is brought into heat-transfer contact with the outer-surface of heating roll 6 to induce heat into the aluminium strip to heat the aluminium strip to an annealing temperature. While progressively moving the lower-surface 2 of the aluminium alloy strip 1 is brought into heat-transfer contact with the outer-surface of heating roll 7, followed by bringing the upper-surface 3 of the aluminium alloy strip 1 into heat-transfer contact with the outer-surface of heating roll 8. Both heating rolls 7 and 9 are rotatable in a first direction, i.e. clockwise or counter-clockwise, and heating roll 8 is rotatable in an opposite second direction. By adjusting the transport speed or line speed of the moving aluminium alloy strip 1 it receives a heat treatment by soaking for a certain time at a pre-defined annealing temperature sufficient to achieve annealing required for the subject aluminium alloy. Soaking times are typically in a range of up to 1 minute, and preferably up to 30 sec. To enhance the heat input into the aluminium alloy strip 1, it can be pre-heated prior to being brought into contact with the first rotatable heating roll. The pre-heat can be achieved by various heating means, for example by using an inductive heating device 13. To assist in the temperature control of the aluminium alloy strip and to modulate heat-loss of the strip, heat shields 12 or screens 12 can be used. The screen is reflective at least on the side facing the heating roll and the moving aluminium sheet and reflects the infra-red radiation of the moving aluminium alloy strip or by adsorbing and re-emitting infra-red radiation. The thermal shield or screen also prevents or avoids uncontrolled temperature loss by preventing air currents around the moving aluminium sheet in the space or chamber defined by the rotating heating roll and the thermal shield or screen. Optionally, the heat shield or reflective screen can be provided further with active heating means (not shown). Following the heat-treatment, the aluminium alloy strip 1 is rapidly cooled or quenched in a quenching section by moving or transporting the aluminium strip over cylindrical rotatable cooling rolls 9,10,11, wherein the rotating cooling rolls comprises an outer-surface, such that a surface of the aluminium strip is in heat-transfer contact with the outer-surface of the rotating cooling rolls to remove heat from the aluminium strip and to cool the aluminium strip to a temperature below 100° C., and preferably to about ambient temperature. In this set-up, the aluminium alloy strip 1 is also further cooled by actively spraying water or water-mist via spray nozzles 14 onto either surface 2,3 of aluminium alloy strip to enhance the cooling rate of the strip material. Alternative approaches have been described herein.

FIG. 3 is a schematic representation of another embodiment of the method according to the invention and the apparatus used therein. In this configuration, aluminium alloy strip 1 is being uncoiled and via a cylindrical transfer roll or support roll 15 moved or transported to a heating section comprising three cylindrical rotatable heating rolls 6,7,8 of the same diameter and next transported to a rapid cooling or quenching section (not shown). All three rotatable heating rolls are provided with heat shields 12 or reflective screens 12. In this configuration, rotatable heating rolls 6 and 7 are heated via an external induction source 16, whereas rotatable heating roll 8 is heated by means of electric resistance heating (not shown).

FIG. 4 is a schematic representation of another embodiment of the method according to the invention and the apparatus used therein. Also in this configuration aluminium alloy strip 1 is being uncoiled and via a cylindrical transfer roll or support roll 15 moved or transported to a heating section comprising three cylindrical rotatable heating rolls 6,7,8 and next transported to a rapid cooling section (not shown). All three rotatable heating rolls 6,7,8 are heated by means of electric resistance heating. Optionally the moving aluminium alloy strip 1 can be pre-heated by means of induction heating using induction device 13. In this configuration the aluminium alloy strip 1 while in heat transfer contact with the outer-surface of the first heating roll 6 is further heated by means of induction heating using induction source 16. Such additional induction heating of the aluminium alloy strip 1 can be applied at one rotatable heating roll, but also at or near more of the rotatable heating rolls.

This kind of arrangement would be particularly suitable for processing high-strength alloys, for example but without limiting to these examples, aluminium strips from AA2XXX-series aluminium alloys for aircraft applications or AA7XXX-series aluminium alloys for aircraft or automotive applications, as these aluminium alloys contain high amounts of alloying elements requiring a longer soaking time at solution heat treatment temperature. With the current regular industrial equipment available for continuous annealing and quenching of aluminium alloy strips, this longer soaking time obliges to severely reduce the speed of the line (exit speed of the strip), typically by up to about 70% compared to the line speeds used for AA6XXX-series aluminium alloys, making these continuous annealing lines very costly to operate for manufacturing these high-strength aluminium alloys. In the approach of this invention, this can be done very much easier and more cost effective by increasing the diameter of the heating rolls or by adding one or more heating rolls while maintaining a high line speed of the moving aluminium alloy strip.

The invention is not limited to the embodiments described before, and which may be varied widely within the scope of the invention as defined by the appending claims. 

1. Method for the heat treatment of a moving aluminium alloy strip, the aluminium strip has an upper-surface and a lower-surface, the method comprising moving the aluminium strip over at least two rotating heating rolls, wherein the heating rolls comprise an outer-surface, such that a surface of the aluminium strip is in heat-transfer contact with a part of the outer-surface of the heating rolls to induce heat into the aluminium alloy strip to heat the aluminium alloy strip at an annealing temperature, and comprising moving the aluminium alloy strip over a first rotating heating roll followed by moving the aluminium strip over a second rotating heating roll such that alternating the upper-surface and the lower-surface of the aluminium strip are in heat-transfer contact with the outer-surface of the rotating heating rolls.
 2. The method according to claim 1, wherein the outer-surface of the heating rolls is coated with a composite diamond coating.
 3. The method according to claim 1, wherein the outer-surface of the heating rolls is coated with a ceramic coating, preferably selected from the group consisting of titanium nitride, tungsten carbide, and chromium nitride.
 4. The method according to claim 1, wherein the aluminium alloy strip is moving while one surface is in heat-transfer contact with a rotating heating roll and heat-loss of the other surface of the aluminium alloy strip is modulated by the presence of a screen.
 5. The method according to claim 1, wherein the aluminium alloy strip following the heat treatment is quenched to below 100° C.
 6. The method according to claim 1, wherein the aluminium alloy strip following the heat treatment is quenched to below 100° C. by moving the aluminium alloy strip over at least one rotating cooling roll, wherein the rotating cooling roll comprises an outer-surface, such that a surface of the aluminium alloy strip is in heat-transfer contact with the outer-surface of the rotating cooling roll to remove heat from the aluminium alloy strip to cool the aluminium alloy strip at a temperature below 100° C.
 7. The method according to claim 1, wherein the annealing temperature is in a range of 400° C. to 590° C.
 8. The method according to claim 1, wherein the heating roll has a diameter in a range of 1 to 3 meters.
 9. The method according to claim 1, wherein the heating roll is made from a metal, preferably selected from the group of cast iron, steel, stainless steel, copper, copper-based alloy, and aluminium alloy.
 10. The method according to claim 1, wherein the aluminium alloy strip has a thickness in a range of 0.3 mm to 4.5 mm, and preferably of 0.7 mm to 4 mm.
 11. The method according to claim 1, wherein the aluminium alloy strip has a composition within the AA2XXX-, AA5XXX, AA6XXX- or AA7XXX-series aluminium alloys.
 12. Facility for implementation of the method according to claim 1, characterised in that it comprises: a heating-section comprising two or more rotatable heating rolls adapted to move or transport in use an aluminium alloy strip while in heat-transfer contact with the outer-surface of the rotatable heating roll to induce heat into the aluminium alloy strip to heat the aluminium alloy strip at an annealing temperature; and a quenching section for rapid cooling or quenching of the aluminium alloy strip from the annealing temperature to below 100° C.
 13. Facility according claim 12, further comprising one of more screens positioned to face the side of the aluminium alloy strip that is not in heat transfer contact with the outer-surface of a rotatable heating roll to modulate the heat-loss of a moving aluminium alloy strip.
 14. Facility according to claim 12, wherein the outer-surface of the heating rolls is coated with a composite diamond coating. 